Autism is a behaviorally-defined neurodevelopmental disorder with a CDC-reported prevalence of approximately 1 in 166 children in the US and future societal costs estimates up to $43 billion per year. Despite the urgency to uncover the biologic basis of autism, research progress has been slow. Although both genetic and environmental factors are thought to be involved, none as yet have been reproducibly identified. The metabolic basis for autism has received much less research attention despite the fact that chronic biochemical imbalance is often a primary factor in the development of complex disease. The metabolic phenotype of an individual reflects the influence of endogenous and exogenous factors on genotype. As such, it provides a window through which the interactive impact of genes and environment may be viewed and relevant susceptibility factors identified. Based on our recent discovery that children with autism exhibit abnormal methionine and glutathione metabolism, we hypothesize that the observed metabolic imbalance results in increased oxidative stress and impaired methylation capacity that may contribute, in part, to the development and clinical manifestations of autism. We further hypothesize that the abnormal metabolic profile will be associated with increased frequency of genetic polymorphisms that functionally affect methionine and glutathione metabolism. Aim 1 will determine whether the severity and specificity of metabolite imbalance is associated with quantitative measures of cognition and behavior and whether the metabolic profile has positive predictive value as a biochemical test for autism to support the behavioral diagnosis. Aim 2 will prospectively investigate whether the abnormal metabolic profile is detectable in high risk toddlers before the behavioral testing is possible as a means to expedite early intervention and treatment strategies to improve outcome. Mechanistic Aim 3 will establish whether cells from autistic children exhibit evidence of oxidative damage, increased vulnerability to oxidative stress, and/or DNA hypomethylation. Using the metabolic profile as a phenotypic map for the selection of candidate genes, Aim 4 will determine whether autism is associated with specific genetic polymorphisms, gene-gene and gene- metabolite interactions that affect methionine and glutathione metabolism. The knowledge gained from this proposal will add a new metabolic dimension to the diagnosis and clinical management of children with autism. The identification of a metabolic endophenotype associated with increased risk of autism will provide new insights into treatment options and new directions for translational research. The public health significance of this proposal is the clinical translation of these findings into early detection and improved diagnosis, better understanding of autism-related pathology, and new targeted intervention strategies to improve the health and clinical outcome of children with autism.